This invention relates to a focusing control device for a cine camera or the like, which operates to displace the lens so that the lens is automatically and continuously focused on an object, and more particularly to an improvement in such devices which prevents the useless displacement of the lens which is caused by the detection of an infinite focus condition.
An example of a focusing system to which the improvement according to the invention is applicable is shown in U.S. Pat. No. 4,251,145 to Cocron, et al.
In general, an automatic focusing device for a cine camera or the like has a pair of range finding elements which are displaced according to the amount of extension of the photographing lens. When light reflected from an object is equally applied to the pair of range finding elements, the lens is focused on the object. When the quantity of incident light to one of the range finding elements is different from that of incident light to the other range finding element, the lens is defocused, i.e. it is in a front focusing condition or in a rear focusing condition. Accordingly, the lens is moved in a direction so that it is focused on the object. In this case, the weaker output signals of the range finding elements are subjected to integration in order to increase the S/N ratio thereof. Until the integration has been accomplished, the lens moving motor is controlled according to the result of the preceding integration.
In the case where an object is infinitely distant, in an active type automatic focusing device as described above, the quantity of light which is reflected by the object and received by the range finding elements is substantially zero, and accordingly it is impossible to carry out the above-described integration. Accordingly, heretofore, when the integration is impossible, the focusing position of the lens is moved so that the lens is in the infinitely-distant-object photographing condition. However, if, in this case, there are backgrounds (such as the sky, mountains and clouds) at distant ranges although an object to be photographed is positioned near as shown in FIG. 3, the object is defocused. Furthermore, if a range finding light beam or the like is completely absorbed by an object, then similarly it is impossible to carry out the integration, and accordingly, the object is defocused. Moreover, in the case where a panning operation is carried out as shown in FIG. 4, the focusing point is varied frequently because there are different persons and backgrounds at different distances, and accordingly the resultant picture is unsatisfactory in quality.